Between the solid crystalline phase and the fluid melt, intermediate phases occur in certain substances which combine structural and dynamic properties of both the ordered crystalline state and of the unordered melt state. Although these phases are fluid, they have, for example, optical properties which are characteristic of the majority of crystalline, but also partially crystalline substances such as, for example, birefringent. These are referred to as intermediate phases (mesophases) or alternatively liquid-crystalline phases. These phases can be obtained by varying the temperature, in this case one refers to thermotropic liquid crystals, or in solution by varying the concentrations. Hereinafter, we are only concerned with thermotropic liquid crystals.
The existence ranges of these intermediate phases are generally characterized by, for example, transition temperatures, determined calorimetrically or by means of a polarizing microscope, from the crystalline state to the liquid-crystalline state (glass transition temperature) and from the liquid-crystalline state to the liquid melt (clearing point) (cf. G. Allen & J.C. Bevington, Eds., Comprehensive Polymer Science, Vol. 5, pp. 701-732, Pergamon Press, 1989). If different liquid-crystalline states are present, the set of corresponding transition temperatures is indicated.
The structure of the liquid-crystalline phases is characterized by a different long-range and short-range degree of ordering of the molecules. A distinction is made between nematic phases, smectic phases and cholesteric phases. Cholesteric phases are also known as chiral nematic phases or twisted nematic phases.
In the nematic phase, the molecular centers are distributed without order, while the long axes of the molecules are aligned parallel to one another. This is different to the state in the fluid melt, where the molecule long axes are arranged randomly.
In the smectic phases, a regular arrangement of the molecular centers in space occurs in addition to the alignment order of the nematic phase described above. This regular arrangement can be present along one, but also along two or even three, independent spatial axes. These phases are nevertheless fluid.
In the cholesteric phase, layers of nematically arranged molecules are arranged one on top of the other so that a continuous helical variation of the alignment direction of the molecule long axes is produced. The molecules thus form a helical structure with the period p. The cholesteric phase thus has a helical structure. Of the liquid-crystalline phases, it has particular properties (cf., for example. Bergmann-Schaefer, Experimental-physik, Volume III: Optik, 7th Edition (1987), pp. 560-567, or de Vries, Acta crystallogr., (1951), 4, 219-226).
Thus, it is known that the cholesteric phase of liquid-crystalline substances in a macroscopic alignment in which the helical axes are arranged parallel to one another and perpendicular to the surface reflects light incident parallel to the helical axis (angle of incidence 0.degree.) in a wavelength range .lambda..sub.ref, which is determined by the period p, also known as the pitch, the refractive indices and the birefringence of the helical material (de Vries, H.I., (1951), Acta crystallogr., 4, 219; Meier, G. in: Physical Properties of Liquid Crystals, ed.: Meier, Sackmann, Grabmeier, Springer-Verlag, (1975), 9-11).
The reflected light is circular-polarized, the direction of rotation of the reflected light corresponding to the direction of rotation of the helical structure of the cholesteric phase (Jacobs, S.D., J. Fusion Energy, (186), 5(1), 65). However, no reflections occur at .lambda..sub.ref /2, .lambda..sub.ref /3, etc, as in other systems which have a periodic structure (for example X-ray diffraction at crystals or vapor-deposited coatings for optics) known as Bragg reflections. If the helix axis is tilted by the angle .alpha., the reflection wavelength shifts in accordance with the Bragg law to .lambda..sub.ref .times.cos (.alpha.) (Eberle, H.J., Miller, A., Kreuzer, F.-H., Liquid Crystals, (1989), 5(3), 907).
Other known liquid-crystalline phases having a helical structure are, for example, the S.sub.A * and S.sub.c * phases. In the S.sub.A * phase, the molecules within the layers are in an arrangement which is analogous to the S.sub.A phase. From layer to layer, however, the molecules are twisted with respect to one another. A twisting of the smectic layers is thus obtained (J.W. Goodby et al., A New Molecular Ordering in Helical Liquid Crystals, J. Am. Chem. Soc. (1989) 111, 8119-8125; T.J. Bunning et al., Bilayer structures in cholesteric, cyclic-siloxane liquid crystals, LIQUID CRYSTALS (1991), 10(4), 445-456). In the S.sub.c * phase, which is used, for example, in ferroelectric liquid-crystalline displays, the tilt angle of the molecules forms a helical structure. For a suitable periodicity of the helix, selective reflection of light also occurs in this phase. These phases have the same optical properties as the cholesteric phase, which was described above.
German Offenlegungsschrift DE 3 920 420 describes a process for the production of optical components on the basis of polymeric supports containing at least one dye, using linear-polarized light. In the process described therein, a system S, comprising the polymeric organic support T and at least one photoisomerizable dye F, is varied in a specific manner by a macroscopic, light-induced structuring by incidence of linear-polarized light, depending on the polarization direction and the morphology of the organic support.
Polymeric organic supports T which are suitable for the above process are non-helical and non-twisted nematic polymers. These are irradiated with linear-polarized light, and the effects are detected in transmission.
Optical elements which reflect linear-polarized light at an angle not equal to 0.degree. to the direction of the incident light (dielectric layers) are known.
It is likewise known that the combination of a layer of a cholesteric liquid crystal and a quarter-wave plate acts as a reflector for linear-polarized light.
Therefore, it is an object of the present invention to provide optical elements which reflect light incident perpendicular to the surface in a linear-polarized manner parallel to the direction of incidence of the incident light. Another object of the present invention is to provide optical elements containing only one optically effective component based on liquid-crystalline substances. Still another object of the present invention is to provide an optical element which by illuminating a helical liquid-crystalline substance with linear-polarized light in such a manner that the substance is subsequently aligned so that it exhibits at least one reflection band of linear-polarized light on illumination. A further object of the present invention is to provide a process for preparing the optical elements which reflects light incident perpendicular to the surface in a linear-polarized manner parallel to the direction of incidence of the incident light.